Balance and proprioception impairment, assessment tools, and rehabilitation training in patients with total hip arthroplasty: a systematic review

Background Osteoarthritis and subsequent total hip arthroplasty (THA) lead to damages to hip joint mechanoceptors, which in turns lead to impairments in proprioception. One of the abilities mainly affected by an altered joint proprioception is balance. The aim of this work was to investigate the balance and proprioception impairments, current assessment tools, and rehabilitation training after THA. Methods A systematic literature revision was conducted on PubMed, Web of Science and Cochrane databases. Articles reporting balance and proprioception impairments, current assessment tools, or rehabilitation interventions were included. Methodological quality was assessed using the Downs and Black checklist. A total of 41 articles were included, 33 discussing balance and proprioception assessment, and 8 dealing with training. Data related to type of surgical approach, type and timing of assessment protocols, assessment instrumentation, and type, volume and duration of the rehabilitation training were extracted from each study. Results Thirty-one studies were of high quality, 2 of moderate quality and 8 of low-quality. Literature review showed an improvement in balance following THA in comparison with the pre-operative performance, although balance abnormalities persist up to 5 years after surgery, with THA patients showing an increased risk for falls. Balance training is effective in all the rehabilitation phases if specifically structured for balance enhancement and consistent in training volume. It remains unclear which assessments are more appropriate for the different rehabilitation phases, and if differences exist between the different surgical procedures used for THA. Only two studies assessed proprioception. Conclusion Balance and proprioception show impairments up to 5 years after THA, increasing the risk of falls. However, patients with THA may benefit of an adequate balance training. Further research is needed to investigate the gaps in balance and proprioception assessment and training following THA surgery. Supplementary Information The online version contains supplementary material available at 10.1186/s12891-021-04919-w.

Labanca et al. BMC Musculoskeletal Disorders (2021) 22:1055 interventions for joints replacement may compromise part of the joint structures and surrounding components, with joint mechanoceptors being the most affected structure by THA surgery. The damages to these mechanoceptors lead to impairments in proprioception, i.e., to a lack or to abnormal afferent signals informing the brain of joints' position and movement. Abnormal proprioceptive signals do not only affect sensory function, but also motor control, since sensory information is essential for movement programming [4]. For these reasons, patients undergoing joint replacement surgery show both sensitive and motor abnormalities [2,5,6]. One of the "abilities" mainly affected by abnormal proprioception is balance, and its impairments may compromise the quality of life [7][8][9], and the risk of falls [10]. In THA patients, balance deficits may be persistent after surgery [11,12], leading to an increase in the risk of falls [13], especially in the first year after surgery [14].
The abnormalities in proprioception may also affect the biomechanics of functional movements. In fact, if afferent information is lacking, movements cannot be controlled across the whole range of motion of the joint. It is not surprising that following THA, patients show gait abnormalities up to 1 year after surgery [2]. In particular, a reduction of gait velocity and stride length have been reported, accompanied by a reduction of the time of single limb support, and a reduced range of motion in the sagittal plane [2]. Different surgical approaches for THA implant may affect hip proprioception in different ways. In fact, while the direct anterior approach (DAA) affects only hip joint capsule [15], other procedures, such as the lateral and posterior techniques, may also affect muscles and tendons [15] causing higher damages to proprioceptors. However, while the effect of different surgical approaches on hip biomechanics and clinical outcomes has been widely studied [16][17][18][19], the entity of proprioception compromise in this setting has not been extensively explored so far.
Moreover, few studies have been reported by previous systematic reviews on the benefits of balance training following THA [20],and it is not clear which kind of exercises should be adopted and how these should be differentiated in the different rehabilitation phases after THA surgery.
Having clear information on the magnitude of balance and proprioception impairments following THA would be of high clinical relevance, to ascertain which kind of balance and proprioception training interventions should be adopted, and how deficits and improvements throughout rehabilitation should be assessed. Therefore, the current study has three main objectives. The first is to provide an updated systematic review on balance and proprioception impairments following THA surgery. The second is to investigate how balance and proprioception deficits are measured. The third objective is to investigate how balance and proprioception are trained during the rehabilitation following THA. These three points will be investigated by differentiating results according to the surgical approach used for THA implant.

Population and diagnosis of interest
This review includes all studies on balance and proprioception impairments, assessment or training after THA for degenerative arthritis.

Search strategy and inclusion criteria
A systematic review of PubMed, Web of Science, and Cochrane database was performed. The inclusion criteria were: (1) articles published between June 1, 2000, and August 31, 2021; (2) patients with THA for degenerative arthritis were recruited, (3) assessment of balance and/or proprioception, and (4) training of balance and/or proprioception. No limitations were placed over the type of surgical procedure used for THA. Non-English language publications, review articles, conference proceedings, editorials, case-studies, letters, methodological studies, animal studies, and cadaveric studies were excluded. A time frame of 20 years was chosen for two main reasons. The first, is related to the fact that methodological literature suggest to write reviews based on nearly the last 5-10 years to be considered up-to-date [21,22]. Then the suggested timing was extended to 20 years to include a broader number of evidences. The second reason is related to the to the fact that studies older than 20 years may have been based on surgical procedures, as for example high invasive approaches requiring long healing time, which are no longer adopted in clinical practice to date [23]. As a consequence, the post-surgical assessment, in particular in the early post-surgery was mainly based on surgery-related consequences (e.g, pain, bleeding, …) rather than on the assessment of functional abilities having an impact on the quality of life [23].
The terms and key words used for the research strategy were: (total hip arthroplasty OR THA OR hip replacement OR hip prosthesis) AND (balance OR propriocept* OR postural control) located within the title and/or abstract and/or keywords. The character * was used to include in the research both the terms proprioceptive, proprioception, and proprioceptors. Reference lists and citations of the included articles were manually screened to identify additional studies of interest. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist was employed to guarantee review methodological quality (Additional file 1: Appendix 1). The study was registered in PROSPERO (CRD42020213412).

Selection process
Three independent reviewers (FC, FB, NS) performed the research and selection of the papers. Duplicates were removed and then the titles and abstracts of all the studies were reviewed to determine their eligibility. In case of disagreement in the appropriateness of the paper, a fourth author (LL) was consulted to determine abstracts inclusion; in that case, the full-text version of the paper was retrieved and screened to determine the eligibility of the paper. Then, the full-text of all the eligible articles was retrieved and assessed to further verify inclusion and exclusion criteria meeting. Selection process of the papers' selection is represented in Fig. 1.

Risk of bias assessment
The Quality Index Checklist by Downs and Black [24] was used to assess the risk of bias of the studies. The full version of the Downs and Black Checklist [24] was used to assess randomized and non-randomized clinical trials, while the modified version was used to assess all the other studies. The number of items of the modified version is reduced from 27 to 14 (1,2,3,5,6,7,10,11,12,16,18,20,21,22) to eliminate questions that are applicable only for intervention studies. Each study was divided by quality according to the percentage of met items: low quality (< 60%), moderate quality (61-74%), or high quality (≥75%) [25]. Detailed information on the risk of bias assessment is reported in Additional file 2: Appendix 2. The reviewers assigned the judgment and the Fleiss' Kappa measure was used for assessing the reliability of agreement between the two reviewers by the use of an independent observer. This measure calculates the degree of agreement in classification compared to what would be expected by chance, and it is scored  [26].

Outcomes of interest and data synthesis
Data were extracted by three independent reviewers (FC, FB, NS), and in case of disagreement a fourth reviewer (LL) was consulted. Data extracted from studies focused on balance and proprioception assessment were: (1) authors of the study; (2) characteristics of the patients involved in the study; (3) surgical approach for THA implant; (4) the training intervention performed; (5) the protocol of assessment; (6) timepoints at which the assessments were performed; and (7) clinical results. Data extracted from the studies focused on the training of balance and proprioception were: (1) authors of the study; (2) characteristics of the patients involved in the study; (3) surgical approach for THA; (4) volume, duration and time of the training; (5) the assessments performed and timepoints in which these were performed; (6) the protocol of assessment; and (7) clinical results. If relevant data were not reported in any of the studies, the corresponding author was contacted. The data extracted from the studies and included in the qualitative synthesis are reported in Table 1 and Table 2.

Deviations from the study protocol registered in PROSPERO database
Two adjustments with respect to the original study protocol registered on PROSPERO database should be mentioned. First, the Downs and Black checklist was adopted instead of previously planned tool as it can be adapted to both intervention and non-intervention studies, and thus allowed to use a single tool for all the studies included in the present review. This choice was not made on the basis of the results of methodological assessment, rather on the willingness to provide better clarity in the reporting and the discussion of the risk of bias assessment in the text of the manuscript. The assessment with the Downs & Black checklist and the assessment with previously planned tools was performed and the results showed comparable outcomes between the tools. Detailed information on the assessment and the comparison between the tools is reported in Additional file 3: Appendix 3. In addition, in this review there was also an attempt to analyse differences between patients undergoing different surgical approaches for THA, and this was not planned in the original study protocol. However, it was deemed appropriate to extract also these data since this information is highly useful for clinical practice and for the setting-up of the rehabilitation programs.

Search results
According to the PRISMA flow-chart (Fig. 1), a total of 1780 articles were retrieved from the initial literature search. After removal of duplicates and of studies not meeting inclusion and exclusion criteria, 48 articles were considered eligible for a full-text review. Five articles [27,28,[29][30][31][32] were excluded because the results from patients with THA were mixed to results of patients operated on for other orthopaedic conditions. One study was excluded because it was a reliability study [30], and another one was excluded because it did not report postsurgical data on balance assessment [33]. At the end of the selection protocol, 41 articles were included and evaluated for systematic review; of these, 33 were assessment studies [11,13,, while the remaining 8 were rehabilitation training studies [65][66][67][68][69][70][71][72]. A summary of the data extracted from each article is reported in Table 1 and Table 2.

Studies quality and risk of bias
The scores of the modified Quality Index for the included studies ranged between 64.2 and 100%. A total of 2 studies were of moderate quality (score 60-74%), and 31 were of high quality (score ≥ 75%), while the remaining 8 articles were of low quality (score < 60%). The risk of bias score for each individual study is reported in Additional file 2: Appendix 2. A graphical representation of the risk of bias across all the studies reporting balance and proprioception impairments and assessments is provided in Fig. 2, while risk of bias across studies investigating rehabilitation training for balance is represented in Fig. 3. The majority of the papers included in this review (31 out of 41) were of high quality, thus showing a low risk of bias. However, regarding articles on balance and proprioception assessment, sources of high risk of bias were observed regarding the items 5, 12 and 21 of the checklists, which referred to the reporting of confounders among the patients recruited (reporting bias), the representativity of the participants recruited with respect to the whole population (external validity) and the common source population between cases and controls (selection bias), respectively. In addition, since a high percentage of study did not clearly report the required information, some concerns arise regarding items 11 and 22, i.e., the representativity of the participants which were asked to participate in the study with respect to the whole population (external validity) and the recruitment of cases and controls in the same period of time (selection bias). THA with anterior, anterolateral or posterior approach.
Postural stability was assessed using a pressure measurement mat (RSscan International Co.,Belgium).
Balance was assessed using the Berg Balance Scale (BBS) Balance confidence was assessed with the Activitiesspecific Balance Confidence Scale (ABC) Functional reach test. Participants were asked to stand barefoot with 4 ft placements: shoulder width stance (SWS), feet side-by-side stance (SSS), tandem stance with affected limb in the front (AFS), and tandem stance with non-affected limb in the front (SFS).
They were instructed to stand as still as possible with both arms at their sides and eyes staring at a target 5 m away in front of them.
In each foot placement condition, three trials lasting 30 s were recorded with 15 s resting interval between trials. The BBS is comprised of 14 tasks and is scored on a 5-point scale (0-4). The ABC is a 16-item self-report measure of a person's confidence in performing various activities of daily living without falling or experiencing a sense of unsteadiness.
One day before surgery, at 2 weeks, 6 weeks, 3 months, 6 months, and one year after surgery.
When the foot placement changes from shoulder width stance (SWS), side-by-side stance (SSS), and non-affected limb in the front stance (SFS) to affected limb in the front stance (AFS), the subjects tended to increase their postural sway after surgery and the progressive increment of CoP sway path length (CoPR THA with an anterolateral approach. The centre of mass trajectories was assessed using a 8 camera system (Vicon Motion Systems Ltd., Oxford, UK) and a conventional biomechanical model, the Plug-In-Gait full-body. Participants were asked to rise from a seated position to a standing position as fast as possible five times consecutively, with the arms placed across their chest. The test was performed twice.
One month prior to THA and 1 year postoperatively. in the horizontal plane) was given and the subjects had to balance on the moving platform to regain their equilibrium in the given position. In this case, the rigid plate performed damped free oscillation; the damping corresponded to the subject's balancing ability.
The testing was performed in the sequence of standing on both limbs, standing on the non-affected limb (dominant limb for controls) and standing on the affected limb (nondominant limb for controls). Each subject underwent 9 tests.
Lehr's damping ratio was calculated as the balancing capacity in response to a sudden unidirectional perturbation.
Prior to and at 6 weeks, 12 weeks and 6 months after THA.
In the case of direct-lateral and antero-lateral exposure, Lehr's damping ratio significantly decreased compared to the preoperative values at 6 weeks postoperatively, but it increased steadily after-wards. Lehr's damping ratio while standing on the affected limb was significantly lower -even at 6 months postoperatively -than that of the control group. In the case of posterior exposure (with the joint capsule preserved), Lehr's damping ratio continuously increased in the postoperative period and corresponded to that of the control group at 6 months after total hip arthroplasty. ?

Dynamic postural stability
was assessed using a stabilometric force platform (PROPRIO 5000 machine, Perry Dynamics, Decatur, IL, USA) Patients were placed on the platform with their feet shoulder width apart, knees slightly flexed, and center of mass centered over the center of the board by a single examiner.
A 6-in. piece of rope was placed in the patients' hands and they were instructed to hold this in front of them to remove the effect of the upper extremities on balance. An ultrasonic sensor was placed at the L5-S1 junction and this transmitted the patients' position in space. After the double-limb testing was complete, three 1-min tests in each single-limb stance were performed with the side being tested first being randomly assigned. Each trial finished when one of the following criteria was met: 1 min had elapsed, the patient exceeded 3 in. of movement in 0.25 s, the patient moved greater than 5 in. from the starting point, the patient let go of the rope, the patient moved their feet, or the patient asked to stop. Step test. Subjects were asked to stand as still as possible with feet at shoulder width for 120 s on the force platforms recorded at 120 Hz the ground reaction forces and moments. The patients were asked to fix a target located 3 m in front of them at eye level. For the timed up and go test, the patient had to rise from a standard arm chair, walk as fast as possible until he reached a stool placed 10 ft ahead of him, contour it, and come back to the chair and sit back.
During the functional reach test, the patient was standing still, shoulder flexed at 90°and the other arm on the side. We asked the patient to reach as far as possible ahead of him with his index finger without lifting the heels from the ground and the maximum distance reached (in centimetres) was measured with a ruler. The step test consisted of five consecutive rises and descents from an 18-in. step (using the operated leg to climb up and keeping the same leg on the step when going down) that were performed as fast as possible. We recorded the time to accomplish this task.
Before surgery, 3, 6, and minimum 12 months No difference was found preoperatively and at all follow-ups among HR, large-diameter head THA, and control subjects during the quiet standing test as shown by similar total path length of the COP in all groups. There were no differences between the prosthesis groups for the timed up and go test. We found differences between the two prostheses groups for only two tests: the functional reach and the step test at the advantage of patients undergoing HR and those undergoing large-diameter head THA, respectively.

THA with trans-gluteal approach
Balance in the form of trunk pitch (forwards-backwards) and roll (side-to-side) movements was assessed with the SwayStar balance system (Balance International Innovations GmbH) Participants were asked to perform quiet stance, gait, gait over barriers, stairs and sit-to-stand tasks.
Before surgery and at 4 and 12 months after surgery There was a progressive improvement at 4 and 12 months for gait and sit-to-stand tasks parameters. By 12 months, the values approached those of the control group. Trunk pitch (forwards-backwards) and roll (side-to-side) velocities were less stable when walking over barriers as was roll for the sit-to-stand task, indicative of a residual deficit of balance in THA patients.
Merle et al. 14 patients (F 6, M 8; mean age between 57 and 85 y) THA with postero-exterior (11) and anterior (3)  THA and SRA with a posterior surgical approach and uncemented prosthesis.
Postural balance was assessed using a force platform AMTI force plate (Advance Mechanical Technology Inc., MA, USA). All participants were asked to perform two postural tasks.
For the first task, patients were requested to maintain a quiet standing posture on the force platform, with eyes open and feet at shoulder width for 120 s. For the second task, patients had to maintain a one leg stance position for 10 s.
The operated leg was tested twice with an inter-trial resting period of 30 s.

Six months after surgery
During static dual stance, the statistical analyses revealed significantly larger Root-Mean-Square and RMS COM -Root-meansquare (RMS) of the center of pressure (COP)-amplitude in the medial-lateral direction for THA subjects compared to SRA and control subjects. Statistical analysis showed significant dependence between groups and one leg stance completion (P = 0.01). Five of the ten patients in the THA group did not complete the task compared to one for the SRA subject. In the control group all subjects completed the task Nantel et al. Postural balance was assessed using a force platform AMTI force plate (Advance Mechanical Technology Inc., MA, USA) Each participant had to achieve 2 postural tasks. The first task consisted of quiet standing for 120 s with eyes open. In the second task, patients had to hold a one-leg stance position on the operated limb for 10 s. The operated leg was tested twice with a sufficient intertrial resting period. This task was considered successful when the patient was able to stay still on 1 leg for 10 s and was considered unsuccessful if the patient had to touch the ground with the contralateral foot.

Between 5 and 7 months
The statistical analyses for the dual stance task revealed significantly lower larger Root-Mean-Square COP amplitudes in the medial-lateral direction for large diameter head THA and SRA subjects compared to control subjects. No significant differences between groups in There was no significant difference in the ability to complete the one-leg stance task between the 3 groups. The fall rate was surveyed by asking participants if they had fallen in the past year. Falls were defined as "a person falling onto the same level or a lower level on their own, with no external force from another person, loss of consciousness, paralysis from a sudden attack such as stroke, or an epileptic seizure. were treated as binary with success being greater than 10 s and failure being less than 10 s. For the YBT-LQ, the participant was instructed to remain in unilateral stance on the stance platform while pushing the reach indicator in three independent directions (Anterior, Postero-medial, and Postero-lateral) At least 1 year following surgery Women failed single-leg stance at a higher rate than men. Reach distance was different between limbs for all reach directions with greater reach distance on the nonoperative limb for all patients.
Men had a greater reach distance in the ANT and PM directions. THA with posterolateral (10) and anterior (4) approach Standing balance on a double force platform (PF02, Equi+, Aix les Bains, France). The subjects stood barefoot on a double force platform with eyes closed. Control group participants were required to adopt an asymmetrical body weight distribution close to that observed on average for the patients.
The center-of-pressure (CP) and the center-of-gravity (CG) movements were analysed for each limb.

days after surgery
Patient with THA showed greater movements for both plantar and resultant CP displacements, principally along the antero-posterior (AP) axis, a decreased contribution of the hip mechanisms in the production of CP displacements along the medio-lateral (ML) axis, greater resultant CP and CG movements along the AP axis and increased differences between CP and CG along both   (15), and right and left second metatarsal phalangeal joints. Heel switches were affixed to the bottom of the heels of each shoe to assist in data processing of heel strike and toe off identification for temporal-spatial analysis. Participants were instructed to wear their customary walking shoes and to walk as they would normally at their typical walking speed All measurements were obtained in a single session.
The patients were a minimum of 2 months post-surgery (Ten of the THA participants were 2-3 months post-surgery, and one each 5, 6 and 7 months, and one 2 years post-surgery). Balance analysis on both legs found comparable results in the control and resurfacing groups. The weight-bearing both leg balance area was greater in the hip replacement than in either of the other two groups (p < 0.05), and five times greater than in the resurfacing group (p < 0.05). The single leg weight-bearing balance results were significantly better in the resurfacing group, with a balance area half that of the hip replacement group, whether on the operated or the non-operated side (p < 0.001).
In all groups, the difference between left and right monopedal stance was non-significant. Postural tasks: Three retro-reflective markers were placed on the two acromial angles and on the seventh cervical vertebra to detect their motion with the optoelectronic system. The following tasks were recorded: 1) Standing -patients were asked to stand barefoot in standing position with their arms crossed over their chest (feet were positioned parallel on two adjacent force platforms with heels at 10 cm apart and equidistant from the medial edge). Study participants were asked to maintain the standing position for 60 s with eyes open (EO) while looking at a fixed point placed in front of them at two meters distance.
After resting for 5 min, the same test was executed with eyes closed (EC).
2) Stand-to-Sit -patients were seated on an adjustable-height chair with back support, with knees flexed at 100° (feet placed parallel on two adjacent force platforms with heels at 20 cm and equidistant from the medial edges). Patients were asked to stand up and, after 10 s, they were asked to sit down with their arm across their chest returning to the same initial position. Only the stand-to-sit component of the task (STS) was processed due to the inability of several participants to rise from the chair without using their arms.
Before surgery, at three and seven days after.
No between-group differences were found for TUG. BWDSI during STS and standing revealed differences over time in favour of patients with bilateral THA, who showed better symmetry in weight distribution. Shorter CoP path length was observed during standing in patients with unilateral THA, who mainly used their non-affected limb to maintain balance.
Ulivi et al.   It was performed on 1 board (including "path") and with different sensitivity of platform (B100 and B60). The board showed "paths" for displacement of centre of feet pressure. Patients could observe certain position of the centre of feet pressure (COP) on the screen. It was visualized as a cursor and the tasks were to achieve the targets successively displayed on the screen during displacement of the body.
The subject's position during dynamic tests was upright with feet placed parallel and 20 cm apart. TUG: During the test, patients were to rise from a chair, walk a distance of 3 m, make a turn of 180 ° having crossed a designated line and return to the chair. Recording the time of performing the task was initiated by the "start" command and stopped the moment a patient returned to the sitting position with the back resting against the chair. Patients were instructed to do the task as quickly as possible, but at the maximum speed at which the patient could walk safely without running THR with anterolateral approach A balance platform and a oneleg standing test (OLS) were used to assess static balance. At the beginning of the rehabilitation process.
Significant imbalance in the sagittal plane during normal standing EO and EC positions were found in the THR group. No significant differences in the measured parameters were found during tests in tandem, the second form of tandem and one-leg standing positions in the groups. The mean time of standing on the operated limb in the THR group during the OLS test was significantly shorter than that in the control group.      The experimental group showed a statistical improvement in the mean Biodex overall stability index at 6 weeks and 12 weeks interval compared to the initial.
The control group did not show any improvement across time.
Trudelle-Jackson et al.   Regarding studies on balance and proprioception training, sources of high risk of bias were observed regarding the items 2, 8, 9, 15 and 24, i.e., the clear description of the outcome measures (reporting bias), at least an attempt to measure adverse events (reporting bias), the description of patients lost in follow-ups (reporting bias), blinding of the assessors (internal validity bias) and the concealment of the randomized assignment to study groups of both patients and health-care staff (selection bias), respectively. No information or no information mixed with sources of high risk of bias were observed regarding items 11, 12 and 13, which referred all to as external validity of the study (i.e., the representativity of the participants which were asked to participate in the study with respect to the whole population, the representativity of the participants recruited with respect to the whole population, and the representativity of the staff, places and facilities used for the study with regards to those usually used for patients), and the items 14, 22 and 27, i.e., the blinding of patients with respect to their own group assignment (internal validity bias), the recruitment of cases and controls in the same period of time (selection bias), and the statistical power of the study, respectively.

Static balance / postural stability
Static balance, also referred to as postural stability, was assessed by double-or single-limb stance tasks on force platforms in 11 studies, which analysed centre of pressure (COP) velocity and displacement [11,[38][39][40][41][42][43][44][45][46][47]. Four of these [41][42][43]47] were performed in three groups: patients with THA, patients with Hip Resurfacing (HR), and healthy age-matched controls. Two studies were performed on patients undergoing a regular size THA, and these reported a higher COP displacement in patients with THA compared to HR and healthy agematched controls during the double-limb standing task [42,47]. The other two studies which involved patients with large-diameter THA reported no differences in COP path length and displacement between controls, HR and THA patients [41], or a lower medial-lateral displacement in HR and THA compared to controls [43]. A better balance performance in HR and controls compared to THA was found also during the singlelimb stance task [42,47], and no differences between the three groups were found in the study involving patients with large-diameter THA [43]. Timing of assessment was heterogenous between studies, and ranged between 5-and 15-months following surgery. One study reported an increased COP displacement in THA patients 12 days after surgery in comparison to healthy matched controls [38]. The last study [39] compared 3 groups of patients within 2 months after surgery, who received 3 different surgical approaches (posterior, anterior and Röttinger approches) and a group of healthy matched controls. No significant differences between approaches were found for fulfilment of the single-limb balance tasks. However, subjects operated on with the anterior or Röttinger approach showed higher average COP displacement speed and path length than controls. Subjects operated on through the posterior approach showed no significant differences from controls.
Two studies compared COP displacement during double-limb stance, in open-and closed-eyes condition [ 44,45]. Both studies reported in the closed-eyes condition a higher displacement of COP in THR compared to healthy age-matched controls, at 6-and 12-months post-surgery, and in a single assessment between 24-and 36-months post-surgery [44]. A significant improvement was reported between 6-and 12-months following surgery [45].
A lower displacement of the COP was reported in patients undergoing unilateral THA in comparison with patients undergoing bilateral THA [11]. Two studies having no control group reported a reduction of postural sway in the post-operative compared to pre-operative period during double-limb stance, and a higher sagittal sway compared to medial-lateral sway during single-limb or tandem stances, independently from the examined limb [ 40,46].
One study assessed stability by means of pre-determined protocols of an instrumented platform (Biodex Balance System), and it reported an improvement in the parameters related to postural stability at 6 months after surgery compared to pre-surgery data [48]. Another study, using another instrumented device (PROPRIO 5000 machine) found that with single-limb testing of the operated limb, the HR group performed better than patients undergoing standard THA, and there was a trend for HRs to perform better than the overall THA population, but not better than large-headed THAs [49]. In the same study [49], no differences were found for postural stability in double-limb stance between patients with HR, patients with THA femoral head > 32 mm, patients with THA femoral head ≤32 mm, and the control group.
Four studies assessed the time that patients were able to spend in a single-limb stance position [50][51][52]. Two of them found a significantly shorter time in THA group compared to healthy controls for the operated side [ 51,52], while another study without a control group of healthy participants, reported a shorter time in the operated compared to the non-operated side [ 50]. The fourth study reported a higher failure rate during the stance position in female patients compared to male patients, more than 1 year after surgery [63].
Two studies assessed the between-limbs loading difference while standing [11,53]. A higher loading of the non-operated limb was reported in patients with unilateral THA [11,53], while loading was more symmetrical in patients with bilateral THA [11].
In the study by Merle et al. [37], THA patients were asked to stand on two force plates in two conditions: a comfortable position, or with the requirement to load the operated limb. In the first condition, the operated limb was less loaded than the healthy limb, while in the second condition load distribution was close to symmetry, but a higher displacement along the medial-lateral axis was found for the trajectories measured under the healthy limb than under the operated limb.
Finally, balance in terms of COP displacement seems to be not affected by the position of the elbow (straight or flexed) for holding a crutch at a mean of 4 days after surgery, even if the elbow flexed increases shoulder loading [ 35].

Dynamic balance
Two studies investigated dynamic balance during the performance of a 5-times-sit-to-stand [54] and a 5-timessquat [55] on force plates. Within 1 month after surgery, THA patients showed a higher asymmetrical loading and higher anterior-posterior (AP) and medial-lateral (ML) COP displacement during squatting movements when compared with healthy age-matched controls [55]. Asymmetry and COP displacement were found to be significantly different from healthy controls 1 year after surgery during sit-to-stands movements [54].
Two studies investigated dynamic balance during walking by means of force platform and 3D motion analysis [56,57]. One study reported that, compared with healthy age-matched control subjects, THA patients showed greater frontal plane (FP) center of mass-center of pressure (COM-COP) inclination angles and smaller sagittal plane angles which improved postoperatively, but remained significantly different from healthy controls [57]. The other study [56] found no differences during single-limb-support phase for within and between group comparisons for the COM projection over the base of support. During the double-limb support phase for the healthy age-matched older adults serving as a control group, the vertical projection of the COM during doublelimb-support was held on average laterally towards the leading limb side, while the individuals with THA held their COM on average toward their operated limb both limb lead conditions.
One study investigated dynamic balance in response to a sudden medial-lateral perturbation in three groups of THA patients undergoing the three different surgical approaches [58]. It was found that, in the case of directlateral and antero-lateral exposure, Lehr's damping ratio significantly decreased compared to the preoperative values at 6 weeks postoperatively, but it increased steadily afterwards. Lehr's damping ratio while standing on the affected limb was significantly lower -even at 6 months postoperatively -than that of the healthy age-matched control group. In the case of posterior surgical approach, Lehr's damping ratio continuously increased in the postoperative period and corresponded to that of the control group at 6 months after THA.
Balance in response to sudden platform inclinations and translations was investigate also in another study [34], which found no differences between THA patients 4 months after surgery and healthy age-matched controls.
Three studies investigated dynamic balance during a step test. One year after THA no differences for least number of steps, Step Test for non-surgical side and Step Test for the THA side, between patients operated with lateral approach and patients operated by the anterior approach were found [59]. The other study [41] reported that patient operated with the large-diameter head THA were able to complete the task approximately 3 s faster than the HR group (p = 0.001). A few numbers of steps were reported in THA patients 5 years after surgery [52].
One study investigated dynamic balance in the immediate post-surgical period during a biofeedback test based on COP displacement, and found that in THA patients Time and distance of COP displacement were higher, the performance of the task was slower and the displacement of the COP was higher compared to controls [52].
The last study [36], investigated postural sway by means of a portable system embedded with gyroscopes, applied over the low back, during the performance of gait and gait over barriers, stairs climbing and sist-to-stand tasks. It was found a progressive improvement of balance between 4 and 12 months after surgery for gait and sitto-stand tasks parameters. At 12 months post-surgery, THA patients results approached those of healthy control group participants, however, trunk pitch (forwards-backwards) and roll (side-to-side) velocities were less stable when walking over barriers as was roll for the sit-to-stand task, indicative of a residual deficit of balance in THA patients.

Hip joint position sense
The study by Jo et al. [60] investigating proprioception by means of hip joint position sense reported no differences between patients undergoing THA for osteoarthritis or after hip fracture 3 months after surgery, while Wang et al. [61] reported that THA patients had higher mean absolute angle error than HR patients at 6 and 12 months from surgery; after 1 year and up to 36 months, both groups had similar absolute angle error. In either study, results were not compared with a control group of healthy age-matched participants.

Clinical scales and other tasks
Berg Balance Scale was used in three studies. Warenczackc et al. [52] reported no differences between THA patients and healthy age-matched controls at 5 years after surgery. Chang et al. [40] found that Berg balance test decreased significantly after 2 weeks from THA surgery and improved gradually thereafter, reaching the highest score at 6 months. Jo et al. [60] reported that 3 months after surgery BBS scores of patients undergoing THA for degenerative arthritis were significantly higher than patients undergoing THA because of fractures scores.
Activities-specific Balance Confidence Scale (ABC) was used in two studies [13,59]. No differences were found at 1 year from surgery when patients operated on by a direct anterior and direct lateral approaches were compared [59]. Moreover, ABC scale was found not to be a predictor of falls in THA patients [13].
Timed-up-and-go was used in four studies. No differences have been reported between patients with unilateral and bilateral THA [11], patients undergoing either a direct superior approach or a posterolateral approach for surgery [64], as well as in patients undergoing largediameter THA or HR. [41] However, THA patients were significantly slower than healthy age-matched controls [ 41,62].
Functional reach test was used in three studies [ 40,41,62]. Patients with THA reached shorter distance than healthy age-matched controls when studied between 3 and 5 years after surgery [ 41,62]. Accordingly, another study reported no significant improvements between 2 weeks and 1 year after surgery [40]. Patients undergoing HR had better performance from 3 to 12 months after surgery when compared to large-diameter heads THA patients [41].
One study reported a higher reaching distance during the Low Quarter Y-balance test in male compared to female patients, more than 1 year after surgery [63].
Risk of falls was assessed in two studies. The first study the assessment was performed by means of the Falls Risk for Older People in a Community Setting (FROP-Com) tool; the authors found no differences between patients undergoing THA with the direct anterior approach and patients undergoing THA with the direct lateral approach [59]. The second study, performed the assessment by means of the Brief BEST-test in two groups of THA patients, undergoing either a direct superior approach or a posterolateral approach for surgery [64]. A reduction in the risk of falls was observed in both groups between 1-and 3-months following surgery.
Fall rate was assessed in one study, which reported a 2.8 times higher fall rate in THA patients compared to healthy age-matched controls in the ten years after surgery [51].
The 3-m walk and 30-s chair standing tests were used by Wareczack et al. [62] 5 years after THA surgery. Patients with THA performed shorter distance and a lower number of repetitions, respectively, when compared to healthy age-matched controls.

Balance rehabilitation training
Three studies investigated the effects of a training intervention in the immediate post-surgical rehabilitation [65,66,72], one study in the period between 4 and 12 months after surgery [67], and one study in the period between 3 and 6 months after surgery [70]. Four studies reported a higher increase in balance in patients undergoing balance training [65,66,70,72], while the study by Nelson et al. [66] reported a similar between-groups increase in balance following a non-specific balance training.
Two studies investigated the effects of a pre-operative balance training program on the post-operative outcomes. One study found positive effects of the preoperative balance training, based on Tai-Chi practice performed up to week 26 post-surgery [68], while another study found no differences between the groups in the post-surgery [69].
Better balance abilities while walking, in terms of reduced postural sway, were found in a group of THA patients undergoing a strength training protocol in comparison with THA patients undergoing a traditional rehabilitation [71]. This result was observed after a fatiguing test battery 3 months after surgery, while no betweengroup differences were observed at 6 and 12 months after surgery.
All the studies referred to balance or postural stability. The term proprioception was not used.

Balance and proprioception impairment and assessment tools
The first aim of this review was to investigate if patients following THA show impairments in balance and proprioception. The high heterogeneity of studies methodologies and the lack of controls groups of healthy participants, or comparisons with normative data, makes difficult drawing conclusions. However, almost all the studies comparing results of THA patients with healthy controls reports significant differences between-groups, with THA patients have worse balance performance than healthy controls. Impairments have been reported during static [38,39,41,44,47,51,63] and dynamic balance assessments [36,54,58], as well as with the assessment by means of clinical scales or tests batteries [41,51,[62][63][64].Only 4 studies reported no differences between THA patients and controls in balance during the singlelimb phase of walking [56], the Berg-balance scale results 5 year after surgery [62], and in response to balance perturbations [34,58]. Further studies with control groups of healthy participants are needed to better clarify these results.
Regarding proprioception impairments, it should be mentioned that in the only two studies retrieved for this review [60,61], the results of patients with THA were not compared to results of a control group. Further studies are strongly recommended given the essential role of proprioception for both static and dynamic balance performance. It is not possible to draw conclusion on the timing of rehabilitation in which impairments are higher. It seems that a general improvement is reported in the post-operative compared to the pre-operative period [40,46,48]. Some studies with more than one assessment in the post-surgical follow-up reported a balance improvement across time [45,58,64], while other did not [40,46,49]. However, there are many studies reporting balance impairments also years after surgery [40,41,45,51,54,62,63], and persistent differences with healthy controls despite the improvements [57], thus it is likely to think that balance impairments are never completely addressed following THA. These observations are confirmed by the results of the study by Ninomiya et al. [51], which observed a 2.8 times higher fall rate in THA patients compared to healthy controls in the 10 years after surgery. The second aim of this review was to investigate how balance and proprioception are commonly assessed in THA patients. It seems that the most used tasks for balance assessment are those investigating COP displacement or other similar parameters during double-and single-limb stance performed on force platforms or other instrumented devices (22 out of the 32 studies in this review). In general, the use of force platforms for the assessment of static and dynamic balance is well accepted in literature given the high reliability of the instrumentation [73]. In the specific case of patients undergoing THA, the use of force platforms together with other devices, such as camera for motion capture or inertial sensors, may provide also additional information on the kinematic of the operated hip joint and the whole lower limb, which may show peculiar abnormalities or compensations negatively affecting for example the non-operated limb or the whole-body posture. Further research is needed to deeply investigate these aspects and how balance changes in the long-term following THA. Regarding proprioception, it was assessed in only two studies [60,61] by means of hip joint repositioning tasks. These tasks are mainly aimed at investigating the joint position sense. It will be useful adding other tasks to investigate proprioception during dynamic tasks. In addition, further research is needed to understand the extent to which abnormal hip proprioception affects whole-body balance above and beyond the other body functions and structures involved in balance ability. It is not possible to draw conclusion on which tests are more indicated in the different times of rehabilitation because of the heterogeneity of the studies. However, some considerations have to be mentioned regarding some of the other tests used for balance assessment. The assessment of dynamic balance by means of asymmetrical loading during squatting and sit-to-standing, as well as loading during walking, needs to consider that balance is not the only variable affecting the results of the measure. Other factors such as muscle strength, post-operative training of loading symmetry, and fear of loading the operated limb might play a role in the performance of those tasks. The same is for tasks such as the functional reach test, used as a measure of balance. Undoubtfully balance is required for the functional reach test, but also muscle strength of hip, back and in general upper body muscles play an important role for a good performance of the test. At the same time, it should be mentioned that muscle strength might contributes per se to balance abilities. In support of this observation, one of the studies included in the present review reported better balance abilities while walking in the early post-surgery in THA patients undergoing a strength training intervention in comparison with patients receiving a usual rehabilitation [71]. In addition, in another study [63] male patients showed better static and dynamic balance abilities than the female counterpart. The author concluded that this difference might be related to between-groups differences in muscle strength.
Regarding surgical approaches used for THA, it is not possible to draw conclusions regarding the best surgical approach for the performance of double-and single-limb stance, because of the contrasting results when THA, HR or large-diameter-THAs patients are compared [ 41-43, 47, 49]. Similarly, it is not possible to draw conclusions for the other functional tasks, such as the timed-up-and-go, the functional reach test or the stepping tasks, because of the paucity of studies [ 41,62,64]. Similar findings are observed about hip joint proprioception [61], for the differences between patients with monolateral and bilateral THAs [11,53], and for the differences between patients undergoing THA in the traumatic and elective setting [60]. No differences have been found among patients undergoing anterior or lateral surgical approaches in terms of step test performance [59], of confidence during balance task [59], and of the risk of post-surgical falls [59]. Moreover, no differences were found for balance recovery following sudden perturbations in patients operated on using direct-lateral and antero-lateral surgical approaches, in which the dynamic balancing ability continuously improved in the first 6 months postoperatively [58]. Accordingly, no significant differences between approaches were found for the risk of falls in the first 3 months after surgery [64], as well as for fulfilment of the single-limb balance tasks 2 months following surgery [39]. However, while subjects operated on through the posterior approach showed no significant differences from controls subjects, patients operated on with the anterior or Röttinger approach showed worse balance than controls [39]. In the case of joint capsule preserving posterior approach, the dynamic balancing ability showed a more rapidly improvement across timelines compared to the other two exposures, with no differences in the long-term [59]. However, further evidence is required to confirm these observations.

Balance rehabilitation training
Although the limited number of studies and the different methodological approach do not allow a univocal conclusion, it could be stated that balance training seems to be effective in all the phases of THA, pre-operative [68], immediate post-surgery [65,70,72] and mediumlog-term [68,70], if specifically structured for balance enhancement and consistent in training volume. In fact, the two studies reporting no higher benefits in the intervention group, assessed balance following a nonbalance specific training [66], or after a minimal intervention strategy, demanding minimal training effort exercises [69]. Therefore, balance training interventions for patients with THA should be well structured in terms of type and volume. Just one study suggest that an early strength training intervention leads to better balance while walking in the early post-surgery [71], thus it seems beneficial for the prevention of early re-injuries. No conclusion can be drawn regarding the best training options for the different post-surgical phases.

Limitations
The main limitation of this literature revision was that it was difficult to sum up a conclusion for a number of the investigated points since a limited number of studies were available and most of them were excluded during the selection process mainly because of low quality or because data of THA patients were mixed to data of patients with other orthopaedic impairments. Further, in a number of studies there was not a control group of healthy matched participants to make comparisons with normative data. It is paramount for future studies to eliminate sources of bias and improve studies quality. In addition, since it has been reported that balance abnormalities lead to an increase in the risk of falls [13], another point which should be investigated in future studies, is the relationship between the introduction of specific balance training interventions in the rehabilitation following THA, and the long-term effects on the risk of falls. The studies included in the present review did not report this information, thus results should be considered in light of this limitation. Finally, for some of the points discussed in this review few studies exist. Thus, caution is needed before the modification of clinical practice, and it seems thus essential to conduct further high-quality research to increase the amount of evidences.

Conclusion
Even if a firm conclusion cannot be drawn because of the heterogeneity of the studies and the reduced number of evidences, an improvement in balance abilities is observed following THA surgery in comparison with the pre-operative performance, and balance abnormalities persists for year after surgery, with THA patients showing an increased risk for falls. Since it seems that balance training is effective in all the rehabilitation phases if specifically structured for balance enhancement and consistent in training volume, it is not clear if long-term balance impairments are related to the lack of appropriate training interventions, or to the sensory-motor impairments related to THA surgery. It remains unclear which assessments are more appropriate for the different rehabilitation phases, and if differences exist between the different surgical procedure used for THA. Moreover, only two studies exist assessing hip joint proprioception. Further research is needed to better investigate these gaps in balance and proprioception assessment and training in THA patients.